MYTH 21

THAT MELANISM IN PEPPERED MOTHS IS NOT A GENUINE EXAMPLE OF EVOLUTION BY NATURAL SELECTION

David W. Rudge

So is camouflage the reason natural selection favored the dark moths? Probably not. Further work showed these moths don’t spend much of their days on tree trunks. Some other effect of pollution seems to be at work.

—George B. Johnson and Jonathan B. Losos, The Living World (2010)

The phenomenon of industrial melanism refers to a rapid increase in the frequency of dark forms of moths in the vicinity of industrial areas, which occurred in the wake of the Industrial Revolution (about 1760–1840). It was first noticed in the peppered moth, Biston betularia, a common moth known throughout Britain and Continental Europe and named for its pale, speckled appearance. The discovery of a rare dark form of the moth in 1840 near Manchester, England, led naturalists and insect collectors to search for more examples of what was initially regarded as a sport of nature. Over a period of just a few decades, they discovered numerous additional examples of dark peppered moths and also dark forms of many other species. What made it particularly curious was that the rise in the frequency of the dark form in these species was confined to areas in the vicinity of manufacturing centers, where large-scale air pollution had visibly darkened the environment. Geneticists determined that the dark form of the peppered moth was the result of a variation in a single gene, and in an influential paper, John Burdon Sanderson “Jack” Haldane (1892–1964) pointed out that the speed of the apparent spread of this gene implied that it had an enormous selective advantage in the affected environments.1

These considerations led evolutionary biologists to recognize that the phenomenon of industrial melanism was an example of natural selection taking place before their eyes. This becomes obvious when we consider Charles Darwin’s (1809–1882) theory of natural selection with reference to the phenomenon of industrial melanism:

The reader should notice that Darwin’s theory as stated here does not require us to know precisely how the gene responsible for dark coloration affords dark moths a greater survival advantage in polluted environments. The mere fact that dark coloration is inherited, and that this difference is somehow correlated with survival differences, makes this an example of natural selection.

The claim that industrial melanism is an example of natural selection is not in dispute among scientists who research the phenomenon, although precisely why the dark form became more common in affected areas has historically been the subject of a great deal of debate. James William Tutt (1858–1911) is often identified as the first to popularize the idea that the dark form was becoming more common in the affected areas because of the camouflage value of being dark when resting on soot-darkened backgrounds against visual predators, such as birds. And, indeed, when one compares the pale and dark forms of the peppered moth resting on pale lichen-covered bark taken from an unpolluted forest and the two forms resting on soot-darkened bark, this intuition seems obvious.

Early investigators also considered other explanations for why the dark form was becoming more common. Edmund Brisco “Henry” Ford (1901–1988), one of the pioneers of ecological genetics (an experimental branch of evolutionary biology devoted to the study of the genetic basis of evolution by means of laboratory and field studies), was convinced that the phenomenon was more complicated than most biologists assumed. Ford emphasized anecdotal evidence showing that the dark form in affected species invariably appeared to be “hardier.” Precisely what this meant appears to have depended on the investigator, but the consensus was that the dark form was physiologically superior to the pale form—that is, it was better able to tolerate toxins present in the pollutants. Ford concluded that this physiological advantage was the primary reason why the dark form was becoming more common in the affected areas, and that the spread was limited to polluted areas owing to the obvious handicap of dark coloration in unpolluted areas, where the dark form would be quite conspicuous when it rested against a pale lichen-covered background. James William Heslop Harrison (1881–1967), another British naturalist in the early twentieth century, advanced a different explanation. On the basis of experimental studies in which he fed caterpillars with contaminated foliage, he claimed that lead salts present in the soot fallout from industry had mutagenic properties. These investigations caused quite a stir at the time they were published because they represented evidence of so-called Lamarckian inheritance (the possibility that traits acquired by parents can be passed on to their offspring; see Myth 10). Other investigators attempted but failed to duplicate Harrison’s results, which led to allegations that his original investigations were fraudulent.2

Part of the reason why early investigators were initially reluctant to seize on Tutt’s altogether intuitive explanation was that it was not clear that birds were significant predators on the moths, nor that they would have the same difficulty humans have when attempting to spot them against their matching backgrounds. In the early 1950s, Henry Bernard Davis Kettlewell (1907–1979) conducted a series of pioneering field experiments aimed at resolving these issues. His experiments relied on a technique of mark, release, and recapture, whereby he released known quantities of marked pale and dark moths in both polluted and unpolluted areas of the countryside and then attempted to recapture as many as possible by means of light and assembling traps. He reasoned that all things being equal, the recapture rates for the two forms should be quite similar. If, on the contrary, one form was at an advantage in a particular environment (for example, the dark form was better able to escape from bird predators than the pale in a soot-darkened environment), the recapture rate for the favored form would be higher, because more had survived during the interval between release and recapture. And this is precisely what Kettlewell found: in the polluted setting, the recapture figure for the dark form was twice that of the pale form; and conversely, in the unpolluted setting, the reverse was true. Kettlewell also placed moths representing both forms on soot-darkened and pale lichen–covered tree trunks in the two settings and had an associate, Nikolaas “Niko” Tinbergen (1907–1988), film the order of predation from behind a hide (a barrier that hid his presence from the birds). These films documented that a variety of birds do prey on the moths and that they do so with reference to how conspicuous the moth is when it rests on the tree trunk. They also documented the speed with which birds capture moths, explaining why bird predation had been previously unnoticed.3

The intuitive nature of the phenomenon (compared to other examples available at the time) and the elegant simplicity of Kettlewell’s apparently definitive demonstration led to its wholesale adoption by textbooks. By the late 1960s, industrial melanism had already become ubiquitous in American biology textbooks, leading to its designation as the classic demonstration of natural selection.4 This reaction contrasts with that of other researchers on the phenomenon, who while embracing Kettlewell’s general conclusion nevertheless expressed reservations about the conduct of his investigations, such as how Kettlewell released moths onto tree trunks, and his assumption that moths rest on tree trunks in plain sight during the day. Indeed, one can say that much of the research on industrial melanism since Kettlewell’s first investigations in the early 1950s, by him and others, has been an attempt to remedy these perceived problems. It should be recognized, however, that the basic outline of the explanation we associate with Kettlewell has been confirmed by at least eight field studies. An impressive study conducted on two continents documented a similar rise and predictable fall in the frequency of dark peppered moths in Britain and the United States following the advent of clean-air legislation. Moreover, a recent, large-scale six-year predation experiment specifically designed by the late Michael Majerus (1954–2009) to address perceived problems associated with the conduct of Kettlewell’s original investigations provides the most direct evidence yet for the selective role of bird predation. There is simply no doubt among researchers who work on the phenomenon that it is a dramatic example of natural selection that has occurred primarily as a result of differential bird predation. Contemporary research has established that the phenomenon is more complicated than textbooks imply by drawing attention to the role of other factors, such as sulfur dioxide concentrations and differential migration.5

Critics of the standard story we associate with Kettlewell often suggest that recent anecdotal observations of moths resting higher in the canopy, rather in plain site on the surface of tree trunks, completely undermine this example. Scientists who work on the phenomenon draw no such conclusion. This is not because they are being dogmatic in the face of contrary evidence but because an explanation primarily in terms of selective bird predation remains the best account given the available evidence. Chance observations of moths resting elsewhere simply draw our attention to the fact that more systematic research on the life history of the moth is needed. That some outstanding questions remain regarding why the gene responsible for dark coloration confers a selective advantage does not call into question that this is—and remains—a particularly well-documented example of natural selection.

The phenomenon of industrial melanism has nevertheless in recent years become a lightning rod for critics of evolution, who draw attention to differences between textbook accounts written for introductory audiences and the subtleties shared among scientists who write for technical publications. The intelligent-design theorist Jonathan Wells (b. 1942) harps on these discrepancies as indicative of the systematic way that textbook writers misrepresent the evidence for evolution, urging that biology textbooks be equipped with warning labels.6 He is particularly exercised by the use of “staged” photographs, which misleadingly suggest to the reader that the moths are known to rest on tree trunks (ignoring the more obvious explanation that the textbook is simply trying to illustrate how difficult or easy it is to spot the moth when it rests on a matching or contrasting background). Judith Hooper (b. 1949) in a recent popularization all but accuses Kettlewell of committing fraud, a completely baseless accusation.7

As a predictable consequence of these unfounded attacks, there has been a dramatic decline in the use of industrial melanism as an example of natural selection in American biology textbooks since the early 2000s.8 Part of the problem reflects a general limitation of all textbooks. Textbook writers, in consideration of space limitations and their intended audience, present science as briefly and simply as possible. This systematic omission of details regarding the process of science has the unfortunate consequence of portraying the results of science as certain, rather than tentative and the object of continued investigation. It also perpetuates a myth that Kettlewell worked in isolation, but in truth he relied heavily on other colleagues, not to mention a veritable army of amateur naturalists who collected records of the distribution of pale and dark forms.9 It is important to recognize that every fact in science has a story behind it, and indeed it is by comparing the certitude with which a textbook entry presents industrial melanism with what is actually known about the phenomenon that one can begin to appreciate aspects often referred to as the nature of science, such as the tentative nature of scientific knowledge.10